Can somebody advise me please.I have an O-A extension.I also have a multi re-entry permit valid until 13.3.2013.What happens if I re-enter Thailand after that date.

Are you sure you have an extension on an "O-A" or a one year extension of stay based on "O" for retirement? If you enter after the expiraion, most likely you would be stamped in for 30 days.

You have an extension of stay based on retirement.

What happens afteer you enter Thailand after the re-entry permit (and the extension of stay) has expired is that you will no longer have a valid permision to stay/return to Thailand. In other words, you would need to get a new visa from a Thai consulate to return.

There is only one kind of extension based on retirement (obtained at Thai immigration offices). They behave exactly the same if you have started out with an O visa OR an O-A visa. People with an active O-A multiple entry (not extension) receive a new full year's permission to stay upon entering Thailand up until the validity of the original O-A ends.

I'm also not fully sure what the OP has.

You can not enter after that date as your extension would have expired. So you would need a new visa or visa exempt entry. If you obtain a new extension prior to that date you must obtain a new re-entry permit with that new date for later return travel.